#include <iostream>
#include <thread>

/**
 * The Singleton class defines the `GetInstance` method that serves as an
 * alternative to constructor and lets clients access the same instance of this
 * class over and over.
 */
class Singleton
{

  /**
   * The Singleton's constructor/destructor should always be private to
   * prevent direct construction/desctruction calls with the `new`/`delete`
   * operator.
   */
private:
  static Singleton * pinstance_;
  static std::mutex mutex_;

protected:
  Singleton(const std::string value): value_(value)
  {
  }
  ~Singleton() {}
  std::string value_;

public:
  /**
   * Singletons should not be cloneable.
   */
  Singleton(Singleton &other) = delete;
  /**
   * Singletons should not be assignable.
   */
  void operator=(const Singleton &) = delete;
  /**
   * This is the static method that controls the access to the singleton
   * instance. On the first run, it creates a singleton object and places it
   * into the static field. On subsequent runs, it returns the client existing
   * object stored in the static field.
   */

  static Singleton *GetInstance(const std::string& value);
  /**
   * Finally, any singleton should define some business logic, which can be
   * executed on its instance.
   */
  void SomeBusinessLogic()
  {
    // ...
  }

  std::string value() const{
    return value_;
  }
};

/**
 * Static methods should be defined outside the class.
 */

Singleton* Singleton::pinstance_{nullptr};
std::mutex Singleton::mutex_;

/**
 * The first time we call GetInstance we will lock the storage location
 *      and then we make sure again that the variable is null and then we
 *      set the value. RU:
 */
Singleton *Singleton::GetInstance(const std::string& value)
{
  if (pinstance_ == nullptr)
  {
    std::lock_guard<std::mutex> lock(mutex_);
    if (pinstance_ == nullptr)
    {
      pinstance_ = new Singleton(value);
    }
  }
  return pinstance_;
}

void ThreadFoo(){
  // Following code emulates slow initialization.
  std::this_thread::sleep_for(std::chrono::milliseconds(1000));
  Singleton* singleton = Singleton::GetInstance("FOO");
  std::cout << singleton->value() << "\n";
}

void ThreadBar(){
  // Following code emulates slow initialization.
  std::this_thread::sleep_for(std::chrono::milliseconds(1000));
  Singleton* singleton = Singleton::GetInstance("BAR");
  std::cout << singleton->value() << "\n";
}

int main()
{
  std::cout <<"If you see the same value, then singleton was reused (yay!\n" <<
            "If you see different values, then 2 singletons were created (booo!!)\n\n" <<
            "RESULT:\n";
  std::thread t1(ThreadFoo);
  std::thread t2(ThreadBar);
  t1.join();
  t2.join();

  return 0;
}